Terbium activated rare earth oxyhalide phosphors containing ytterbium for reduced afterglow

ABSTRACT

Phosphors of reduced afterglow having the inclusion of a selected impurity in the phosphors lattice. More specifically, rare earth oxyhalide phosphors of reduced phosphorescence comprising lanthanum and gadolinium oxyhalide phosphors activated with terbium including the selected impurity ytterbium.

United States Patent 191 Rabatin et al.

[111 E Re. 28,592

[ 5] Reissued Oct. 28, 1975 TERBIUM ACTIVATED RARE EARTH OXYHALIDEPHOSPHORS CONTAINING YTTERBIUM FOR REDUCED AFTERGLOW Reissue of:

[64] Patent No.: 3,666,676

Issued: May 30, 1972 Appl. No.: 100,448

Filed: Dec. 21, 1970 [52] US. Cl. 252/30l.4 H [51] Int. Cl. C09K 11/24;C09K l 1/46 [58] Field of Search 252/30l.4 H

[56] References Cited UNITED STATES PATENTS 3,546,128 l2/l970 Rabatin252/30L4 H 3,607,770 9/l97l Rabatin 252/30l.4 H

OTHER PUBLICATIONS Blasse et al., Investigation of Tb ActivatedPhosphors" Philips Research Reports 22(5), 1967, pp. 481-504.

Primary ExaminerJack Cooper Attorney, Agent, or Firm.lohn F. McDevitt;Henry P. Truesdell; Frank L. Neuhauser [57] ABSTRACT Phosphors ofreduced afterglow having the inclusion of a selected impurity in thephosphors lattice. More specifically, rare earth oxyhalide phosphors ofreduced phosphorescence comprising lanthanum and gadolinium oxyhalidephosphors activated with terbium including the selected impurityytterbium.

2 Claims, N0 Drawings 1 TERBIUM ACTIVATED RARE EARTH OXYI-IALIDEPI-IOSPHORS CONTAINING YTTERBIUM FOR REDUCED AFTERGLOW Matter enclosedin heavy brackets I: appears in the original patent but forms no part ofthis reissue specification; matter printed in italics indicates theadditions made by reissue.

BACKGROUND OF THE INVENTION The present invention relates to rare earthoxyhalide phosphors activated with terbium, and particularly, to suchrare earth oxyhalide phosphors having reduced afterglow orphosphorescence.

The rare earth oxyhalides activated with terbium are finding increasinguse as phosphors in X-ray intensifier screens and similar radiographicdevices, replacing past conventional phosphors of less desirableproperties. For example, LaOBrzTb has about a 50 percent greaterabsorption of incident X-rays and about three to four times greater blueemission than the previously used CaWO Currently intensifier screens areprovided with photographic films which tend to fog if a phosphor is usedhaving an afterglow or long persistent phosphorescence of appreciableintensity. With the use of these long afterglow phosphors, any movementof the film relative to the phosphor screen immediately after exposurewill result in blurred images. Attempts to eliminate the problem ofblurred images in X-ray intensifier screens have been generallyunsuccessful because any reduction in duration and intensity ofthephosphor afterglow normally has been at the expense of the overallphosphor efficiency.

The present invention eliminates the problem of blurred images byproviding rare earth oxyhalide phosphors with significantly reducedafterglow without an I appreciable loss in phosphor efficiency.

Accordingly, it is an object of the present invention to provide a rareearth oxyhalide phosphor activated with terbium having a reducedafterflow.

It is a further object of the present invention to provide aterbium-activated lanthanum or gadolinium oxychloride or oxybromidephosphor including a selected impurity to achieve reduced afterglow.

Further objects and attendant advantages will be apparent from thefollowing summary and detailed description.

SUMMARY OF THE INVENTION The present invention relates toterbium-activated rare earth oxyhalide phosphors having a selectedimpurity for reducing the phosphors afterglow. More specifically, thepresent invention relates to rare earth oxyhalide phosphors includingytterbium as an impurity and having the general formula:

wherein M is an element selected from the group consisting of La and Gd;X is an element selected from the 5 group consisting of Cl'and Br; and wis from 0.0005 to 0.03 moles per mole of the selected oxyhalide; and yis from 0.00005 to 0.005 moles per mole of the selected oxyhalide.

o DETAILED DESCRIPTION OF THE INVENTION lected impurity, ytterbium, isincorporated in the phosphor lattice in order to reduce the afterglow ofthe phosphor without any appreciable decrease in the overall phosphorefficiency.

The following are a few of the rare earth oxyhalide phosphors includedin this invention:

The ytterbium impurity addition of the present invention is made by theoxalate precipitation method. This method is outlined below for thepreparation of the o.9n15 o.oo2 Yb 0.005] 0.0005.

In this first step of the preparation, lanthanum oxide (2,330g), terbiumoxide (5.4g), and ytterbium oxide [(0.0l4lg)] (1.413) are dissolved in amixture of concentrated nitric acid (3,055ml) and water (ll,6l5ml). Thismixture is then coprecipitated with 10 percent oxalic acid (40,600ml) toform the oxalates of lanthanum, terbium and ytterbium. The oxalates arethen fired to effect the chemical conversion of the oxalates to themixed oxides'of lanthanum, terbium and yt- 40 terbium at 1,000C for 2hours in open boats.

The mixed oxides of lanthanum, terbium and ytterbium (2,230g) areblended thoroughly with ammonium bromide (1,440g) and fired 2 hours at425C in covered boats to chemically form the phosphor, terbiumactivatedlanthanum oxybromide containing ytterbium.

num oxybromide containing ytterbium. Molten potassium bromide serves asa crystal-growing medium in this firing in order to producewell-crystallized terbiumactivated lanthanum oxybromide containingytterbium. The material is then washed to remove the potassium bromide.It is subsequently dried in an oven for 2 hours at C, and then siftedthrough 325 mesh.

The effect. of ytterbium on the reduction of afterglow in LaOBrzTb isshown in Table I.

TABLE I The Effect of Ytterbium on the Reduction Of Afterglow Propertiesof LaOBrzTb TABLE l-continued The Effect of Ytterbium on the Reductionof Afterglow Properties of LaOBnTb,

Relative Moles of Phosphor Phosphorescence Intensity YtterbiumBrightness In Volts After Indicated Per Mole Under X-ray Time in Sec. ofPhosphor Excitation 0.01 l 9 52 TABLE I] per cm and at a sensitivity of0.05 volts per cm. The phosphor was exposed to the X-rays for 10 secondsafter which the X-rays were turned off and the decay measurements weremade.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

l. A rare earth oxyhalide phosphor of reduced afterglow having thegeneral formula:

wherein M is an element selected from the group consisting of La and Gd;

Effect of Ytterbium on the Afterglow Intensities Of Various Rare EarthOxyhalides Activated with Terbium Moles of Phopsphorescence IntensityYtterbium In Volts After Indicated Phosphor Per Mole Time in Sec.Composition of Phosphor l 3 52 LaOCl:T 0 0000 0 .0000 .28 .18 .12 .02 g5] 9 .0002 .23 .09 .04 .00 .0010 .17 .03 .01 .00 GdOBrzTb 00000 0 .0000.31 .25 .18 .00 5] .0010 .24 .02 .00 .00 GdOClzT 0 0000 .0000 .37 .15.10 .01 l); Show .0010 .25 .00 .00 .00

MEASUREMENTS OF PHOSPHORESCENCE DECAY All decay measurements were madewith an X-ray generator set at 90KV peak and at a milliampere setting togive volt constant output as read on an image storing oscilloscope whenan 5-1] photomultiplier was set at 800 volts. 0.8 gram of phosphor waspressed into a plastic sample holder of mm diameter. The sample wasplaced about 3 inches above the generator target. The photomultiplierwas set about 4 inches above the sample holder. The decay curves wererecorded on the oscilloscope face with the photomultiplier set at 1400volts and the oscilloscope set at a sweep of 5 seconds X is an elementselected from the group consisting of Cl and Br; w is from 0.0005 to0.03 moles per mole of the selected oxyhalide; and y is from 0.00005 to0.005 moles per mole of the selected oxyhalide. 2. A rare earthoxyhalide phosphor of reduced afterglow according to claim 1 wherein:

M is La X is Br w is 0.002 y is 0.0005.

1. A RARE EARTH OXYHALIDE PHOSPHOR OF REDUCED AFTERGLOW HAVING THEGENERAL FORMULA:
 2. A rare earth oxyhalide phosphor of reduced afterglowaccording to claim 1 wherein: M is La X is Br w is 0.002 y is 0.0005.